Magento2: add custom quote extension attribute - magento2

Question

I am trying to add an extension attribute to a quote through a custom REST API, and I would like to retrieve this info later.
But when I try to get the value, is always null.
The code will explain better than me.
etc/di.xml
<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
[...]
<preference for="Wow\Orderplatform\Api\Data\CartInterface"
type="Wow\Orderplatform\Model\Cart" />
</config>
etc/extension_attributes.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
<extension_attributes for="Magento\Quote\Api\Data\CartInterface">
<attribute code="order_platform" type="Wow\Orderplatform\Api\Data\CartInterface"/>
</extension_attributes>
</config>
CartInterface
interface CartInterface extends ExtensibleDataInterface
{
const ORDER_PLATFORM = 'order_platform';
/**
* Retrieve order platform
*
* #return string
*/
public function getOrderPlatform();
/**
* Set order platform
*
* #param string $orderPlatform
* #return $this
*/
public function setOrderPlatform($orderPlatform);
}
The implementation:
class Cart extends AbstractExtensibleModel implements CartInterface
{
/**
* {inheritdoc}
*/
public function getOrderPlatform()
{
return $this->getData(self::ORDER_PLATFORM);
}
/**
* {inheritdoc}
*/
public function setOrderPlatform($orderPlatform)
{
return $this->setData(self::ORDER_PLATFORM, $orderPlatform);
}
}
I try to save the extension attribute in a REST API:
$quote = $this->quoteRepository->getActive($cartId);
$extensions = $quote->getExtensionAttributes();
$platformExt = $this->wowCartExtInterfaceFactory->create();
$platformExt->setOrderPlatform($platform);
$extensions->setOrderPlatform($platformExt);
$quote->setExtensionAttributes($extensions);
$this->quoteRepository->save($quote);
No errors in this procedure.
The problem is that I am not able to get back the value from the quote:
$quote = $this->quoteRepository->getActive($cartId);
$extensions = $quote->getExtensionAttributes();
$extensions->getOrderPlatform(); // Always null
Any hints?
Thanks

Answer 1

This is core bug in magento2 tried to do same for some requirement and found
Join extension attributes are not adding to quote results
There seems to be an issue with join extension attributes as per my debugging
ex.
<extension_attributes for="Magento\Quote\Api\Data\CartItemInterface">
<attribute code="original_catalog_price" type="Magento\NegotiableQuote\Api\Data\NegotiableQuoteItemInterface" >
<join reference_table="negotiable_quote_item" reference_field="quote_item_id" join_on_field="item_id">
<field>original_catalog_price</field>
</join>
</attribute>
</extension_attributes>
Hopefully, the patch will release soon.